The present invention relates to radial tires for use on trucks, buses and the like, and more particularly to pneumatic radial tires with consideration given to improved durability of bead portions thereof.
In a pneumatic radial tire having one or more carcass plies, particularly in a tire used under heavy loads after renewing the tread several times, cracks are likely to occur in the vicinity of an end face of a turn-up portion of the carcass owing to a lack of rigidity of a bead portion, thereby presenting a problem.
Therefore, in order to overcome this lack of rigidity of the bead portion, bead portion structures such as those shown in FIGS. 6a, 6b, and 6c are generally adopted. In the figures, reference numeral 11 denotes a rim of a road wheel. A carcass 10 is composed of one ply or a small number of plies (one ply is shown in the figures) whose carcass ply cords extend in a direction substantially perpendicular to the equatorial plane of the tire, a turn-up portion 10A of the carcass 10 being formed by folding back an end portion of the carcass 10 around a bead core 12 from the inside of the tire towards the outside thereof.
A strip of rubberized steel cords 14 and/or strips of rubberized textile cords 16, 18, 20 respectively serving as bead portion reinforcing layers are disposed singly or in a combination in the vicinity of the bead core 12 on the side of the carcass 10 further away from the side where the bead core 12 is provided.
In the situation where a plurality of bead portion reinforcing layers are used, the more axially outwardly a radial outward end (upper end in the figures) of the bead portion reinforcing layer on the turn-up portion 10A side of the carcass 10 is situated, the more radially outward the end of that bead portion reinforcing layer is situated. It should be noted that the terms "axial" or "axially" used herein and in the appended claims refer to directions which are parallel to the axis of rotation of the tire, while the terms "radial" and "radially" refer to directions that are perpendicular to the axis of rotation of the tire. A lower end 16A of the strip of rubberized textile cords 16 terminates in the vicinity of the bead core 12, but the strip of rubberized steel cords 14 and the strips of rubberized textile cords 18, 20 extend further along the carcass 10 up to its body portion 10B.
A stiffener 21 is interposed between the body portion 10B of the carcass 10 and the turn-up portion 10A thereof. This stiffener 21 has a substantially triangular radial cross-section and comprises a hard stiffener 22 disposed in the vicinity of the bead core 12 and a soft stiffener 24 adjacent to the hard stiffener 22. A radially outer edge of the stiffener 21 is located mcre radially upward than the radially outer edge of the strip of rubberized steel cords 14, or the strip of rubberized textile cords 16, 18, or 20.
With this bead portion structure, the rigidity of the overall bead portion is enhanced by virtue of the reinforcing layers, so that the deformation of a rubber portion 32 in the vicinity of the distal end portion of the carcass 10 is suppressed at the time when the tire rotates under a load.
As a result, the distal end portion of the turn-up portion 10A is less likely to exfoliate from the rubber portion 32.
With such a conventional structure, however, if the reinforcing layer (or layers) is strengthened in order to increase the rigidity of the overall bead portion, the end portion of the reinforcing layer becomes a new source of concentration of strain. Hence, the occurrence of cracks originating from the end portion of the reinforcing layer presents a new issue, so that it has not been possible to sufficiently improve the durability of the bead portion.